The invention relates to a developing process for electrophotography which may be used in an electrophotographic copying machine of the wet developing type involving the use of a developing electrode.
An electrophotographic process is generally known in which a drum- or sheet-shaped photosensitive member is used to form a reproduced pattern of an original thereon, which is transferred to provide a plurality of copies. Of the developing technique in the process which converts an electrostatic latent image on the photosensitive member into a visual image, the present invention is primarily concerned with the disposition of the developing electrode relative to the surface of the photosensitive member and the potential applied thereto.
In order to facilitate the understanding of the background of the present invention, an electrophotographic process will be initially described with reference to FIG. 1 which illustrates a conventional electrostatic copying machine. A photosensitive member 1 is formed on the peripheral surface of a drum and comprises a well-known photoconductive layer. The photoconductive material used in the example shown is of a type which may be charged to either positive or negative polarity, even though the characteristic is not always symmetrical. For each copying operation, the photosensitive member 1 undergoes a rotation through approximately one revolution in the counterclockwise direction, and such copying operation may be repeated to provide a plurality of copies. During one copying cycle, the surface of the photosensitive member 1 is uniformly charged by a primary charger 2, and is then imagewise exposed to an image pattern of an original through an optical system 3, thereby forming an electrostatic latent image thereon. When the surface of the photosensitive member which carries the latent image moves into opposite relationship with a developing electrode 4, which also serves as a developing solution dishplate, it contacts a layer of developing solution flowing thereacross, electrostatically attracting toner particles dispersed in the solution to form a visual image on the photosensitive member. Subsequently, the photosensitive member 1 is engaged by a squeeze roller 5 to remove an excessive amount of developing solution therefrom, and then brought into opposing relationship with a transfer charger 6, which transfers a toner image carried on the surface thereof onto a record sheet. Any amount of developing solution which remains on the surface of the photosensitive member is removed by a cleaning unit 7, and any remaining charge is removed by an a.c. charge eliminating charger 8. This completes one copying cycle of the photosensitive member 1.
In general, the full perimeter of the photosensitive member 1 is not used for the formation of an image thereon, but its surface is divided (FIGS. 2 and 3) into an image forming region (which is designated as R), and a non-image region (designated as N), depending on the size of an original being copied. Such division is usually determined by the operational relationship between the surface of the photosensitive member 1, the primary charger 2 and the optical system 3. The above description of the electrophotographic process relates to the operations applied to the image forming region R, while, for the non-image region N, the primary charger 2 is not activated and the optical system 3 does not perform an exposure operation, but an original or the optical system undergoes a returning movement, even though the transfer charger 6, the cleaning unit 7 and the charge eliminating charger 8 operate in a manner similar to that for the image forming region R. The charger 8 performs a charging of the drum surface to the opposite polarity from that of the primary charger in order to eliminate any residual potential thereon. However, in the present example, the potential to which the charging by the charger 8 is performed is purposely increased to minimize the deposition of the toner onto the non-image region N and the developing electrode 4, whereby a potential of the opposite polarity remains on the region N subsequent to the charge elimination.
The function of the developing electrode 4 is to minimize the edge effect, to maintain a proper optical density in the image formed, and to prevent the deposition of the toner particles on to a background portion of the image, by applying a suitable bias potential to the electrode 4. The purpose of the application of the bias potential is to prevent a background smearing of the photosensitive member 1 by attracting a toner, which may otherwise be deposited on a background portion in the image which forms region R, to the developing electrode 4, through the application of a bias potential thereto which exceeds the residual potential in the background portion of the region R.
As is known, a bias potential may be applied either by a forced biasing technique, in which an external voltage source is provided to feed the developing electrode 4, or a floating bias technique, in which the developing electrode 4 is isolated from the ground so that a charge may be induced thereon by the charge of the latent image on the photosensitive member 1. If the developing electrode 4 is merely left floating when the floating bias technique is employed, the bias potential may rise excessively when copying an original which has a large proportion of black area, thereby unfavorably degrading the contrast of the image. To avoid this difficulty, it has previously been proposed to use a constant voltage diode connected between the developing electrode 4 and the ground and which has a characteristic breakdown voltage slightly exceeding the maximum value of the residual potential in the background portion.
One example of the developing step according to the floating bias technique will be specifically described with reference to FIGS. 2 and 3, assuming that the primary charging achieved a potential of +600 volts. The non-image region N is charged to a potential of -100 volts, the bias potential is established at a constant value of +150 volts, and the residual potential in the background region is +100 volts. Referring to FIG. 2, which illustrates the image forming region R disposed in opposing relationship with the developing electrode 4, the surface of the photosensitive member 1 will be charged such that an image pattern area P maintains the potential of +600 volts while a background area G maintains the residual potential of +100 volts. The potential on the developing electrode 4, which is induced by the charge of the latent image, will be approximately an average of the both potentials, but is maintained below a value of +150 volts by a varistor having a characteristic value of 150 volts.
As indicated by arrows, the toner particles are attracted toward the latent image in the region of the pattern area P and are attracted toward the developing electrode 4 in the region of the background area G, whereby the latent image is converted into a high quality visual image in which the background area is not developed.
When the non-image region N is located opposite to the developing electrode 4, the uniform potential of -100 volts on the surface of the photosensitive member 1 will induce a potential on the developing electrode 4 which is somewhat less than that and on the order of about -70 volts, as shown in FIG. 3. As a result, the toner particles disposed in the developing solution will be attracted toward the developing electrode 4, as indicated by arrows.
During each copying operation, the deposition of the toner onto the developing electrode 4 is repeated to cause an accumulation of the toner thereon, with the consequence that the biasing effect during the developing step is decreased in a gradual manner, giving rise to the appearance of a background smearing in the copied image. When the original has a reduced optical density, the deposition of the toner to be background region is particularly notable. Such degradation in the biasing effect of the developing electrode is not limited to the floating bias technique, but similarly occurs with the forced bias technique. In this instance, even if the electrode is disconnected from the source of forced or external bias during the time it is located opposite to the non-image region N, the potential of the opposite polarity on the photosensitive member causes the toner particles to be driven toward the developing electrode, whereby the toner is accumulated thereon in the manner similar to that with the floating bias technique.